If you getting a message like “Certificate with PSE type >SSL Client (Standard)< has been invalid for X days” in your SM21 system log , then you should check STRUST transaction on your SAP system.
I got the same issue on one of our SAP system ;

SM21 system log output

Solution ;

Go to transaction STRUST , find EXPIRED certificate and DELETE it.

Problem will be solved..

Don’t forget that , if you need this certificate then you have to take new one..